VIII RIVERS AND LAKES 325 
continue, and the river would wander from 
one part of its valley to another, spreading 
the materials and forming a river plain. At - 
length, as the rapidity still further diminished, 
it would no longer have sufficient power even 
to carry off the materials brought down. It 
would form, therefore, a cone or delta, and 
instead of meandering, would tend to divide 
into different branches. These three stages, 
we may call those of — 
1. Deepening and widening ; 
2. Widening and levelling ; 
3. Filling up; 
and every place in the second stage has passed 
through the first; every one in the third has 
passed through the second. 
A velocity of 6 inches per second will lift 
fine sand, 8 inches will move sand as coarse 
as linseed, 12 inches will sweep along fine 
gravel, 24 inches will roll along rounded 
pebbles an inch diameter, and it requires 3 
feet per second at the bottom to sweep along 
angular stones of the size of an egg. 
When a river has so adjusted its slope that 
it neither deepens its bed in the upper portion 
